Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer in an IC. This pattern can be transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned.
Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Typically, an excimer laser is used to supply the lithographic apparatus with radiation in the form of pulsed beams of radiation, e.g., high intensity ultraviolet pulses. Large, expensive lens elements can degrade after receiving billions of the pulses. Optical damage can increase with increasing intensity (i.e., light power (energy/time) per cm2 or mJ/ns/cm2) of the pulses. The typical pulse length from these lasers is about 20 ns, so a 5 mJ laser pulse would have a pulse power intensity of about 0.25 mJ/ns (0.25 MW). Increasing the pulse energy to 10 mJ without changing the pulse duration would result a doubling of the power of the pulses to about 0.5 mJ/ns, which can significantly shorten the usable lifetime of the lens elements.
Pulse stretchers can be configured for use with a lithographic apparatus to minimize optical damage and degradation by substantially increasing the pulse length. Increased pulse length is accomplished by creating copies of the pulse, where each copy is separated in time by using an optical delay.
Using known pulse stretching units can require an initial re-calibration of the lithographic apparatus. Also, the pulse stretching units can have no ability to control the size or direction of the beam without additional periodic calibration.
In addition, pulse stretching units can suffer from the generation of dynamic speckle. Speckle is a function of both the pulse duration as well as the Etendue of the beam. Speckle can be caused by a finite pulse length and limited spectral linewidth of partially coherent radiation from the laser. Speckle can cause a micro non-uniformity of the dose on the wafer resulting in a local variation of the size of the imaged features, often referred to as line width roughness (LWR). Speckle can be reduced by stretching the pulse duration over a period of time or by increasing the Etendue of the beam.